Present access systems comprise access products such as add/drop multiplexers and other similar network elements that are typically housed in a network element housing for deployment in a switching station. Many such switching stations are deployed over a wide geographical area and interconnected by means of wires, optical fibers or other means of communication. Such a network element housing may comprise a metallic structure having a number of plug-in racks each having a number of plug-in circuit boards within. For an add/drop multiplexer, at least one of the racks will typically include the core switch which interfaces, in industry parlance, to east and west high speed ports that are connected to high rate digital serial signals, for example, in the synchronous optical network format. For example, the add/drop multiplexer may interface to an OC-3 optical signal at its east and west ports, each OC-3 optical signal carrying 155.520 Mbit/s signals. This corresponds to three OC-1 (optical) signals at a 51.840 Mbit/s line rate, corresponding to three STS-1 (electrical) signals at the same rate. Each STS-1 frame consists of 90 columns and 9 rows of eight-bit bytes, for a total of 810 bytes (6480 bits). With a frame length of 125 microseconds (i.e., 8,000 frames per second), the STS-1 has a bit rate of 51.840 Mbit/s, as mentioned above. The order of transmission of bytes is row by row, from left to right, as shown in FIG. 7 of ANSI T1.105-1991, which is a well known industry standard defining the digital hierarchy, optical interface rates, formats and multiplexing schemes that result in a modular family of rates and formats available for use in optical interfaces generally referred to as the synchronous optical network. There are numerous other ANSI, CCITT, ISO/IEC and other standards listed in the above referenced document that define all aspects of the SONET.
In the SONET specifications, an add/drop multiplexer is a network element that provides access to all N, or some subset M (where M is a standard hierarchical level less than or equal to N), of the STS line signals contained within an OC-N optical channel. The STS signals are added to (inserted), and/or dropped from (extracted), the OC-N signal as it passes through the add-drop multiplexer. For example, an ADM-150 (also known as the 1603 SM) product of the assignee hereof is capable of interfacing OC-3 signals at its high speed ports and "dropping" three STS-1 signals to subscribers while providing simultaneous access to all subscribers. Thus, the ADM-150 is capable of multiplexing and demultiplexing all the digital level zero (DS0) channels in the three STS-1 signals to which it is intended to be connected. A housing for a network element such as an ADM-150 will typically include, as mentioned above, a rack or shelf containing the main switching fabric interfaced to the high speed ports to which are connected line shelves with further lower-level switching devices for demultiplexing and multiplexing at lower hierarchical levels. For an ADM-150, as mentioned, the OC-3 signal may be broken down to three STS-1 electrical signals each carrying up to 672 bytes or DS0 channels per frame. Each STS-1 signal is thus connected to one or more switches as it is demultiplexed down to the desired hierarchical level. For example, this could include, for each STS-1, up to seven shelves with 48 cards each arranged in twelve-card quadrants, each card being capable of transmitting two digital level zero (DS0) channels at 64 Kbit/sec each. This corresponds to 672 DS0s in the seven shelves associated with a particular STS-1 signal. For three STS-1 signals, there can be twenty-one such shelves for a total of up to 2,016 DS0 channels.
As mentioned, assignee's ADM-150 is capable of multiplexing and demultiplexing all the digital level zero (DS0) channels simultaneously. It will rarely be the case that such simultaneous access will be required by all 2,016 subscribers. Even though every single DS0 channel may be connected to individual subscribers, it will therefore most often be the case that the full capacity of the switching fabric of the ADM-150 or any similar switch will not be fully utilized. It may also be the case that a particular user may wish to expand the number of DS0 channels provided while at the same time not needing the full capacity of an ADM-150 switching fabric. It would be desirable for such a user to be able to add capacity without having to incur the extra cost of a completely new add/drop multiplexer.